Polyimide films have been widely used as a base film for flexible circuit substrates. Oftentimes, these films are used with other polyimide films (and metal foils) to form multi-layer polyimide film laminates bonded to metal.
However polyimide films, in many cases, may not have sufficient peel strength (even when an adhesive is used) and can sometimes peel away from a metal foil causing a long-term reliability problem. To improve this drawback, various electrical, physical, or chemical treatments have been attempted to improve the surface of the polyimide. However, those treatments can also have problems such as using caustic (or very acidic) reagents, adding processing time, and increasing labor requirements.
As one method for improving the adhesive strength of a polyimide film, plasma processing (of the film's surface) is disclosed in Japanese Kokai Patent No. Hei 8[1996]-41227. But in this case, the problem remains that an extra processing step is required to employ the plasma treatment process.
In addition, coating polyimide films with a silane coupling agent (as is disclosed in Japanese Kokai Patent No. Hei 6[1994]-336533) is also known. But in this case there are problems. Here, the adhesive strength can sometimes be lowered because the silane coupling agent can decompose during subsequent heat treatment steps used to convert the polyamic acid to a polyimide.
Furthermore, polyimide films containing tin compounds, and having an excellent adhesion property (as in Japanese Kokai Patent No. Hei 4[1992]-261466) are also known. But, the peel strength of these polyimide films can be about 10 N/cm or less, which may not be good enough in many applications.
Finally, thermoplastic polyimides having good adhesion properties (as in Japanese Kokai Patent No. 2003-27014) are known. But, there can be drawbacks in that these polyimides can ‘sink’ during soldering due to the polyimide's thermoplasticity.
As such, there remains a need for polyimide films in general (and methods for making same) to have sufficient peel strength when adhered to a metal substrate. There remains a need to accomplish this at low cost without requiring chemical reagents to be used, additional time and labor.